1. Field of the Invention
This invention is directed to centralizers for use in wellbore operations; and, in one particular aspect to centralizers with movable spring bows, particularly such centralizers which can be used in a relatively small annular space and which can expand in a larger annular space.
2. Description of Related Art
Bowspring centralizers are used to center one tubular member inside a borehole or in another tubular member, e.g. to center a first smaller casing in a second larger casing. Typically centralizers are placed on the exterior of an inner casing and project outwardly therefrom. In many typical situations the annular space between the outer circumference of the smaller casing and the inner circumference of the larger casing is sufficiently large that, with some force, a centralizer on the inner first casing can be moved into the interior of the second outer casing. The centralizer will present some fluid flow restriction in the annular space.
In a variety of situations the annular space is relatively small and it is difficult or impossible to use conventional multi-spring bow centralizers or conventional rigid centralizers. Attempts have been made to fabricate an apparatus from a tubular member with vanes welded on or milled into the tubular to provide standoff of the inner casing from the outer casing, or, in another aspect, standoff of the casing from a wellbore's interior. One such device provides standoff of casing from a wellbore equal to half the difference between vane outside diameter and casing outside diameter. In some situations these apparatuses do not achieve sufficient standoff, e.g. when a wellbore is underreamed or "washed out" to a significantly larger diameter so that the vanes (or bows) have significantly reduced standoff and/or reduced contact with the wellbore interior.
The present inventors have recognized the problems mentioned above and have recognized that it would be very desirable to have a centralizer which is usable in a relatively small annular space and yet which has the ability to function in a larger annular space.